How to tell value of bit [a] (pg. 109) in the Architecture?

How to tell value of bit [a] (pg. 109) in the Architecture?

I am working on creating my Assembler using C#, and I can accurately specify the first 3-bits (left-most) by deciding if the command is an A/C-command. And on the right-side I can clearly determine if/what the [jump] command should convert to and the same for the [dest].